The first VCSELs produced
850-nm radiation with a beam
width on the order of 10 µm.
Because these lasers produced
multiple transverse modes, they
could be used only with multi-mode fiber. By 2002, scientists
were writing in OPN that 1.5- to
1.6-µm VCSELs had evolved into
tunable components for single-mode fiber in local-area and
metropolitan-area networks.
Lasers in any communications system, from long-haul
transmissions to the short-haul
data sharing in a computer center,
must operate within the proper
parameters for the optical fibers in
the system. The lasers must send
out a signal within the optimum
transmission windows of the
fibers with enough power to reach
the receiver or repeater with a
sufficient signal-to-noise ratio, but
not so much as to create undesirable nonlinearities.
Lasers and optical fiber have evolved together, from the first
Nobel Prize-winning concepts in the 1960s to today’s commodity components. Discussions of communications technologies often cover network architectures, network security, new
multiplexing schemes, improved fibers and marketing strategies
for content delivery, and sometimes it’s hard to figure out where
the lasers fit into the overall picture.
The different types of networks—long-haul (
telecommunications), metropolitan, access and interconnects (of
Metal contact Metal contact Upper bragg reflector (p-type) Upper Bragg reflector (p-type) Lower bragg reflector (n-type) Quantum well Quantum well n-substrate Lower Bragg reflector (n-type) n-substrate
Adapted from Wikimedia Commons
Diagram of a simple VCSEL (not to scale).
short-haul data communications
or datacom)—each have their
own requirements for lasers and
modulation schemes, which are
summarized in the table below.
“Access networks” are the communications links between subscribers and their telecom or datacom
service provider—in other words,
the link to your Internet provider,
cable-television supplier or phone
company (think “fiber to the
home” or FTTH).
Long-haul, metropolitan
and access networks
For the long haul, lasers need
power, even though an ecosystem
of repeaters and amplifiers exists to
boost signals over large distances.
Distributed feedback (DFB)
lasers are still the light source of
choice in telecom. They work well
with single-mode fiber, which
long-haul networks require for
its low attenuation. The narrow spectral width of DFB lasers
makes them appropriate for dense wavelength-division multi-
plexing (DWDM), which sends multiple C- and L-band signals
down the same cable.
In long-haul networks, engineers use WDM to increase the
data capacity of fibers that are already in the ground. “People
don’t want to lay new fiber until they really have to,” said Larry
A. Coldren, professor of optoelectronics and sensors at the
University of California at Santa Barbara (U.S.A.). To improve
Network Type
Long-Haul
Metro
Access
Interconnect
Distance (scale)
>100 km
10 km
Laser type
Primarily DFB
DFB, VCSEL
Limited to 20 km by International
Telecommunications Union (ITU)
standard; often < 10 km
Downstream: DFB
Upstream: DFB or Fabry-Pérot
<100 m
VCSEL
Wavelength
1,550 nm (C-band); 1,565
to 1,625 nm (L-band)
1,310 nm;
1,550 nm
Downstream: 1,490 nm
and/or 1,550 nm
Upstream: 1,310 nm
850 nm; 1,310 nm
Modulation scheme
Speed
Multiplexing scheme
Direct or external
10 Gbps
WDM, DWDM or
coarse WDM
Direct
10 Gbps
Coarse WDM
or DWDM
Direct
Downstream: ≤ 2. 5 Gbps
Upstream: ≤ 1. 24 Gbps
WDM
Direct
10GigE; 40GigE and
100GigE standards
expected mid-2010
Governed by Fiber
Channel and
Ethernet protocols
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